Foamed molded bodies made from silicon and use of said produced products

ABSTRACT

Molded foam bodies made of silicone can be produced in large series by means of known injection molding machines for liquid silicone by virtue of the fact that mixtures of ammonium carbonate, and/or ammonium carbaminate, and/or ammonium hydrogen carbonate, in powdered form, and at least one of the two liquid-paste-like components of liquid silicone are produced and the two liquid silicone components are then processed in the known manner, with the proviso that the mold is not completely filled with the dispersion.

FIELD OF THE INVENTION

The invention relates to a process for the production of molded foambodies, from silicone, especially silicone rubber, as well as the use ofthe products produced therefrom.

BACKGROUND OF THE INVENTION

Silicones can be divided according to their areas of application intooils, resins, and rubbers. Silicone oils, which are processed intoemulsions, anti-foaming agents, pastes, greases, and the like, arelinear polydimethyl siloxanes. Silicone resins are more or lesscross-linked polymethyl or polymethyiphenyl siloxanes, whose elasticityand thermal resistance increases with the content of phenyl groups.Silicone resins are processed, for example, to produce lacquers, to coathousehold utensils, and as laminates. Silicone rubbers are masses thatare convertible to the elastic, rubber state, which contain, as basicpolymers, polydiorganosiloxanes, which exhibit groups that areaccessible to cross-linking reactions. Silicone rubbers differ fromother types of rubbers by virtue of the fact that they are not purelyorganic compounds. Their structure imparts their unique properties tothe silicone rubbers. Generally, a distinction is drawn betweenhot-vulcanizing (HTV) and cold-vulcanizing (RTV) silicone rubbers.

Among the cold-hardening RTV silicone rubber masses, a distinction maybe drawn between single and dual component systems. In the case ofsingle component silicone rubber (RTV-1), the mass polymerizes slowly atroom temperature under the influence of the moisture in the air, wherebythe cross-linking occurs as a result of the condensation of SiOH groups,forming Si—O bonds. The SiOH groups are formed as a result of thehydrolysis of SiX groups of a species that occurs intermedially from apolymer with OH groups in terminal position and a cross-linking agent.In the case of the dual component rubbers (RTV-2), mixtures of silicicacid esters and organic stannous compounds are used.

The HTV silicone rubbers represent, in most cases, materials that can bemolded plastically, which contain highly dispersed silicic acid plusorganic peroxides as cross-linking catalysts, and yield heat-resistantelastic silicone elastomers (silicone rubber) after vulcanization attemperatures in excess of 100° C. Another cross-linking mechanismconsists of the addition of Si—H-groups to Si-bonded vinyl groups, bothof which are built into the polymer groups or at their end,respectively. In the case of HTV silicone rubbers, in addition,radiation cross-linking is known. Since 1980, a liquid silicone rubber(LSR) technology has been established, in which two liquid siliconerubber components are vulcanized in injection molding machines by way ofaddition cross-linking.

Liquid silicone rubbers (LSR's), because of their particular materialproperties, open up new areas of application in elastomer processing.Thus, new elastomer-thermoplast composites can expand the spectrum ofsilicone rubbers known thus far.

The technology for large-scale serial production of solid molded bodiesfrom silicone rubber by using liquid silicone is known as injectionmolding (liquid injection molding, LIM). The liquid injection moldingprocess affords the advantage that complex pieces can be configured in aflexible manner, that pre-heating is not necessary, that after setting,no shrinkage of the molded bodies occurs, and less material is needed.

On injection molding machines (that work completely automatically),first, two liquid-paste-like silicone pre-polymers are thoroughly mixedwith one another at room temperature using a static mixer, and thenpressed into a mold under high pressure. As a result of heating themold, because of the addition reaction between the two components, athree-dimensional cross-linking of the silicone pre-polymers to thesolid, fully-molded silicone polymer, to the molded body made ofsilicone, results (see the corporate prospectuses of the firm of WackerBurghausen regarding ELASTOSIL® LR, or GE BAYER Silicones, Leverkusen,regarding SILOPREN® LSR, or Battenfeld Meinerzhagen regarding injectionmolding machinery for liquid silicone). Frequently, theseinjection-molding machines exhibit dosage apparatuses, so-calledmulti-component dosage apparatuses, for the homogeneous admixture ofcolors or other additives, to the two liquid-paste-like siliconepre-polymers.

Molded foam bodies made of silicone, silicone rubber, for example, suchas profiles or stoppers for bacterial culture bottles, and processes fortheir production, are known.

The Japanese patent application Sho 44-461 (461/1969) describesmoldable, sponge-like silicone rubber compositions that contain athermally degradable swelling agent, especially azobisisobutyonitrile.As a result of the degradation of the swelling agent, substances areproduced that are harmful to human beings and must be regarded,therefore, as problematical from the standpoint of environmentalpollution.

The Japanese patent application Hei 10-36544 (36,544/1998) describes amoldable, sponge-like silicone rubber composition that comprises hollowthermoplastic silicone resin particles that are mixed into the liquidsilicone composition whereby during the polymerization, gases developand in this way, produce pores in the molded body. The silicone rubbersponge that is produced in this manner possesses just slight mechanicalstrength; accordingly, this sponge's uses are limited.

The U.S. Pat. No. 6,299,952 describes a moldable silicone rubber spongecomposition, which also comprises thermoplastic resin spherules thatcontain gas.

Likewise, a process is known in which, initially, liquids having a lowboiling point are mixed with silicone pre-polymers that are notcross-linked. At issue in the case of these liquids, for example, aremethylene chloride, HALON® (polytetrafluoroethylene), hepatane, andtrichloroethylene. The mixture obtained is then filled into a mold insuch a manner that the quantity filled makes up just a percentage of themold's volume. The mold is then exposed to elevated temperatures. In theprocess, the liquid evaporates, as a result of which the siliconepolymer swells up and fills the mold completely. As a result of thetemperature increase, the silicone pre-polymer cross-links increasinglyto a silicone polymer, so that a molded foam body that has the contoursof the mold results. Thus far, this technology has not prevailed forlarge-scale serial production of molded foam bodies, since the admixtureof liquids having a low boiling point to silicone pre-polymers isdisadvantageous. The liquids that can be mixed with siliconepre-polymers are either readily flammable, so that work must proceedunder conditions that afford protection from explosions, or they areharmful to the environment, such as the chlorinated or fluorinated chainhydrocarbons, FCHC's.

BRIEF SUMMARY OF THE INVENTION

The underlying technical problem of the present invention consists ofproviding means and processes for the production of molded foam bodiesout of silicone, especially silicone rubber, whereby the molded bodies,when using known processes, especially the liquid injection moldingprocess, can be produced in a simple and cost-effective manner, and theyexhibit sufficient mechanical stability and whereby in the course of theproduction, the disadvantages that are known in the state of the art areavoided.

DETAILED DESCRIPTION OF THE INVENTION

The present invention solves its underlying problem by providing aprocess for the production of a molded foam body out of silicone rubberwhereby a dual-component liquid silicone composition is thermally heatedin the injection molding apparatus, and vulcanized in a cavity of theinjection-molding apparatus that imparts shape, whereby prior to thethermal treatment, at least one liquid silicone component is mixed withan ammonium compound.

Thus, according to the invention, the problem is solved by virtue of thefact that a mixture of an ammonium compound is produced in at least oneof the two liquid-paste-like components of liquid silicone, preferablyof an addition-cross-linking dual component liquid silicone rubber, andthen processed with the aid of injection mold machinery for liquidsilicone. As a result of the thermal treatment of the liquid silicone, asublimation of the ammonium compound occurs, that is, the latter makesthe transition from the solid state to the gaseous state. The gas thatoccurs in the process, or rather, the gaseous reaction products thatoccur in the process, function as swelling agents, so that within thepolymerizing silicone composition, open and/or closed cells arisethroughout the entire mass.

The ammonium compounds that are used preferably according to theinvention, ammonium carbonate, ammonium carbaminate, and ammoniumhydrogen carbonate, are known, non-toxic crystalline chemicals, whichare used, among other things, in the foodstuffs technology as leaveningagents, such as baking powder, for example. The solid ammonium hydrogencarbonate, which is used, preferably, makes the transition to thegaseous phase at ca. 108° C., without becoming liquid. The reactionproducts that occur in the process are carbon dioxide, ammonia, andwater vapor, reaction products, thus, that are frequently encountered inthe environment. Ammonia, which occurs in higher concentrations, and ischaracterized by its pungent odor, can be readily neutralized by drawingit off and making contact with hydrochloric acid. Surprisingly, it hasalso been shown that ammonium carbonate, ammonium carbaminate, andammonium hydrogen carbonate, or rather, the reaction products that occurat higher temperatures, do not intoxicate the (platinum) catalysts ofthe addition cross-linking dual component silicone.

The production of a mixture of the aforementioned ammonium compounds inat least one of the two liquid-paste-like components of the liquidsilicone, for example, the addition-cross-linking dual component liquidsilicone rubber occurs, according to the invention, by virtue of thefact that a powder of these compounds, whose particulate size,preferably, lies in the lower micrometer range, preferably below 50 μm,is homogeneously mixed into at least one component of the dual componentsilicone.

If these ammonium compounds in powder form should be mixed, in ahomogeneous fashion, into a silicone component having a higherviscosity, for example, over 150,000 mPa*s, in the preferred embodimentof the invention, they are pre-dispersed into silicone oil, whoseviscosity lies, to good advantage, between 100 and 3000 mPa*s. Theresultant dispersion that is obtained can then be readily mixed into thesilicone components; it serves as an inert aid to distribution of theammonium compounds and it can later be easily removed again by temperingthe molded foam body.

In a particularly advantageous and particularly preferred embodiment ofthe invention, molded foam bodies can be produced with injection moldingmachines that are equipped with a dosage apparatus for the admixture ofcolors or other additives to both silicone components. The ammoniumcompounds are dispersed into silicone oil, and the dosage apparatus isequipped with this dispersion, so that instead of colored pigments, theammonium compounds are mixed in with both liquid-paste-like siliconecomponents of the addition-cross-linking dual component silicone.

The concentration of the ammonium compounds in the liquid-paste-likesilicone components or in the silicone oil can be varied across wideranges, and it lies between 0.1 and 50% by weight (relative to the totalmass of the silicone to be processed), preferably, especially, between 2and 8% by weight, and most particularly preferably, at 10% by weight. Inthe case of the especially preferred use of a dosage apparatus, a“saturated” dispersion of the ammonium compounds in silicone oil isused, that is, the ammonium compounds are dispersed in a certain volumeof silicone oil until the ammonium compound and silicone oil form ahomogeneous paste, and no clear silicone oil can be seen any longer.

The production of the molded foam bodies from silicone corresponds,substantially, to that of injection molding of solid silicone moldedbodies by means of injection molding machines, with the difference,however, that according to the invention, the injection mold orshape-imparting cavity is not completely filled with silicone into whichthe ammonium compounds are dispersed. The extent of the filling of themold can be varied in wide ranges by means of the dosage technology thatis present in the injection molding machines, and it can be adapted tothe desired properties of the molded foam body: slight filling of themold corresponds to molded foam bodies of little hardness, a morecomplete filling of the mold corresponds to molded bodies having greaterhardness.

The process for the production of molded foam bodies according to theinvention occurs, preferably, by using initial addition cross-linkingsilicone rubber materials, especially addition cross-linking dualcomponent liquid silicone rubber compositions. In a particularlypreferred embodiment of the invention, commercially availablecompositions, such as SILOPREN®LSR are used. In a further embodiment ofthe invention, the process according to the invention can also beperformed by using initial materials made of peroxide cross-linkingsilicone rubber.

Another preferred embodiment of the invention pertains to the moldedfoam bodies made of silicone rubber that are produced by using theprocess according to the invention. In conjunction with the presentinvention, “molded foam bodies” is taken to mean molded bodies havingopen and/or closed cells or pores across their entire mass. Molded foambodies exhibit a raw density that is less than that of the siliconerubber substance that provides the framework. The molded bodies that areproduced according to the invention are distinguished by an excellentmechanical stability.

According to the invention, molded bodies that are produced in this waycan be used as insulation material against thermal or acoustic effects,as packing material, as absorber material against shock and impact, asabsorber material to absorb fluids, especially homopolar fluids, such as(mineral) oil, as an absorber material for gases and/or solvent vapors,as cushioning material, for example, as a mattress, a mattress overlay,or as pillows, as a medical product, for example, as non-adhesive nasal,aural, anal, fistula, or wound tamponade, and as stoppers for bacteriaculture bottles.

Ammonium hydrogen carbonate (the firm of Fluka), having a particle sizeof <50 μm, which is produced by rubbing on a steel sieve of thecorresponding size, is dispersed by adding and stirring in silicone oilhaving a viscosity of ca. 1,000 mm^(2*)s⁻¹ (Dow Coming® DimeticoneFluid) until a homogeneous paste results. The paste is filled into thedosage apparatus of a multi-component dosage apparatus (the firm of2-Komponenten Maschinenbau, Marienheide-Rodt that is provided for theadmixture of color. The multi-component dosage apparatus is an integralcomponent of an injection molding machine for dual component liquidsilicone rubber (firm of Aarburg, Loβburg) on which dual componentliquid silicone rubber by GE BAYER, Leverkusen SILOPREN®LSR 4030 isprocessed to solid molded bodies.

The production of molded foam bodies occurs by virtue of the fact thatthe dispersion of ammonium hydrogen carbonate in silicone oil is addedto liquid silicone as an admixture by means of the dosage apparatus, andsimultaneously, the filling volume of the mold is reduced to ca. 35%.The other process parameters are substantially identical to the processparameters that are used for the production of solid molded bodies. Theproperties of the molded foam body—hardness, porosity, pore size—can beadjusted across wide ranges by suitable selection of the dual componentliquid silicone rubbers, the quantity of dispersion added to themixture, and the filling volume of the mold. The molded foam bodiesexhibit a skin, the structure of the pores is predominantlyclosed-celled.

1. A process for producing molded foam bodies out of silicone rubberwhereby a dual component liquid silicone composition is heated thermallyin an injection-molding apparatus and is vulcanized in a shape-impartingcavity, whereby prior to the thermal treatment, at least one liquidsilicone component is mixed with an ammonium compound, characterized inthat the ammonium compound, prior to the mixing, is dispersed insilicone oil and, by using a dosage apparatus, mixed with the liquidsilicone component.
 2. A process according to claim 1, characterized inthat the ammonium compound is ground to a powder prior to mixing.
 3. Aprocess according to claim 1, characterized in that the ammoniumcompound is selected from the group comprising ammonium carbaminate,ammonium carbonate, and ammonium hydrogen carbonate.
 4. A processaccording to claim 1, whereby the dual component liquid silicone rubberis an addition cross-linking silicone rubber.
 5. A process according toclaim 1, whereby the dual component liquid silicone rubber is a peroxidecross-linking silicone rubber.
 6. A process according to claim 1,whereby the cavity of the injection-molding device, which imparts shape,is not completely filled with the thermally treated components of theliquid silicone.
 7. A molded foam body made of silicone rubber that maybe produced in accordance with a process according to claim
 1. 8. Themethod of using a molded foam body produced according to claim 1 asinsulation material against thermal or acoustic effects, as packingmaterial, as absorber material against shock and impact, as absorbermaterial to absorb fluids, as an absorber material for gases and/orsolvent vapors, as cushioning material, as a medical product, and asstoppers for bacterial culture bottles.
 9. The method of claim 8,wherein said absorber material to absorb fluid is a homopolar fluid. 10.The method of claim 9, wherein said homopolar fluid is mineral oil. 11.The method of claim 8, wherein said cushioning material is selected fromthe group consisting of a mattress, a mattress overlay, or a pillow. 12.The method of claim 8, wherein said medical product is a non-adhesivenasal, aural, anal, fistula, or wound tamponade.
 13. A process accordingto claim 2, characterized in that the ammonium compound is ground to apowder having a particle size below 50 μm.
 14. A process according toclaim 2, characterized in that the silicon oil has a viscosity between100 and 3000 mPa*s.
 15. A process according to claim 2, characterized inthat the ammonium compound is dispersed in the silicon oil tosaturation.
 16. A process according to claim 15, characterized in thatthe ammonium compound is selected from the group comprising ammoniumcarbaminate, ammonium carbonate, and ammonium hydrogen carbonate.
 17. Aprocess according to claim 15, whereby the dual component liquidsilicone rubber is an addition cross-linking dual component liquidsilicone rubber.
 18. A process according to claim 15, whereby the dualcomponent liquid silicone rubber is a peroxide cross-linking dualcomponent liquid silicone rubber.